This invention relates to top-bottom pincushion correction circuits and more particularly to apparatus for providing an unbalanced top-bottom pincushion correction signal.
As is well known in the cathode ray tube scanning art, the displayed raster of a cathode ray tube is composed of successively spaced horizontal scan lines forming a substantially rectangular-shaped raster. Although the raster is desirably rectangular under normal operation conditions, it is well known that such a desirable condition fails to exist in almost all instances and the raster is distorted in a manner which has become known as top-bottom pincushion distortion.
As is well-known, top-bottom pincushion distortion is indicated by the departure of the horizontal scan lines from a straight-line configuration. Specifically, the horizontal scan lines at the top half of the raster tend to bow downwardly toward the center while the horizontal scan lines at the bottom half of the raster tend to bow upwardly toward the center of the raster.
Also, it is well-known, many modern television receivers have picture tubes mounted near the floor or at least below the normal level of viewer observation. Thus, the average viewer looks down at the raster on the picture tube which has the effect of making normally straight lines near the top of the raster appear bowed inwardly at the center. At the bottom of the raster, the normally straight lines appear to be bowed outward or barrelled at the center of the raster.
One known prior art technique for overcoming the above-described distortion problems was to utilize a movable bias magnet whereby a magnetic field was employed to set up a field in the pincushion transformer to compensate for the distortions. In another method, a DC current was added to the ramp-like waveform of the pincushion correction voltage applied to the vertical windings of the deflection yoke in an effort to provide a desired upbalance in the resultant raster of the cathode ray tube.
Although the above-mentioned methods provided some relief and were reasonably successful, they did leave much to be desired. For example, altering the magnetic flux with a magnet or adding a DC current to effect unbalance of the pincushion correction waveform tends to undesirably alter the crossover point or the point whereat the waveform shifts from one polarity to the other. As a result, the total raster is affected rather than only the positive or negative portion of the waveform which requires correction.